This invention relates to novel, binder-containing granulated silicon carbide products for use in manufacturing highly dense silicon carbide ceramic objects and to silicon carbide ceramic objects produced using the novel granulated silicon carbide products.
Due to its chemical and physical properties, silicon carbide is an outstanding material for many sectors of industry, for example for the semiconductor industry, the automotive industry, for chemical plant engineering, for the machine tool industry, etc. The properties of silicon carbide include high resistance to oxidation and corrosion, high thermal conductivity at high temperatures, a low coefficient of thermal expansion, high thermal shock resistance, heat resistance, hardness and abrasion resistance. It is particularly suitable for manufacturing highly dense articles for many industrial applications, for example highly dense sintered articles.
Various processes are known for producing sintered articles. The oldest process is hot-pressing, in which SiC powder is mixed with small quantities of additives which promote sintering, for example boron or aluminium, and sintered in graphite molds under mechanical pressure at high temperatures of up to about 2000.degree. C. under a protective gas atmosphere. A further process is pressureless sintering, in which submicronic silicon carbide powder, likewise with added sintering aid, is cold-pressed and then sintered pressureless at temperatures of up to 2000.degree. C. in vacuo or under a protective gas atmosphere. A third process is hot-isostatic pressing of SiC. For this purpose, an article cold-formed from silicon carbide is enclosed gas-tight in vacuo in an envelope of silica glass or refractory metal and compacted under a high gas pressure at temperatures of up to 2000.degree. C. In addition, hot-isostatic redensification of silicon carbide sintered pressureless is also possible.
The sintering of silicon carbide alone, however, frequently leads to sintered articles having a high porosity and a density far below the theoretical density of silicon carbide (3.21 g/cm.sup.3). Such porous sintered silicon carbide articles are highly susceptible to oxidation, especially at elevated temperatures, and their mechanical strength is low. In the prior art, the production of sintered silicon carbide articles is therefore carried out with added sintering aids and/or binders, since sintered articles having a better density and better oxidation resistance can be obtained in this way. Sintering aids which are used in many cases include a few per cent of aluminium oxide, metal oxides such as thorium oxide, yttrium oxide, lanthanum oxide or cerium oxide, boron, metallic silicon, tungsten carbide or the like. Examples of binders used in the prior art include organic resins such as phenolic resins or furfuryl resins, organic polymers such as, for example, methylcellulose, polyvinyl alcohol, high-molecular weight organosilicon compounds, namely organopolysiloxanes, organopolyborosiloxanes and polysilanes. The use of oxidic sintering aids or of the above types of binders for the manufacture of high-performance ceramics from silicon carbide is disadvantageous, however, since it is impossible to prevent oxidic and other impurities (for example decomposition products of the binder) from remaining in the resulting sintered articles. Oxidic impurities are especially undesirable in SiC-based high-performance ceramics. Moreover, even if the aforementioned silicon-organic compounds are used as binders, only ceramic products are obtained whose flexural breaking strength is insufficient for high-performance ceramics. Like the nature of the silicon carbide powder used, the additives which are added, especially the binders, also exert a decisive influence on the ceramic product. Therefore, there remains a need for granulated silicon carbide products having a defined composition and defined properties, which facilitate the manufacture of highly pure and highly dense high-performance ceramics from silicon carbide.